Photomultiplier tubes are used in a wide variety of fields as optical sensors employing the photoelectric effect. External light entering the photomultiplier tube passes through a glass bulb and strikes a photoelectric surface, releasing photoelectrons. The emitted photoelectrons are multiplied by successively impinging on dynodes arranged in a plurality of stages. The multiplied photoelectrons are subsequently collected by an anode as an output signal. External light entering the photomultiplier tube is detected by measuring this output signal (see Patent References 1-3, for example).    Patent Reference 1: Japanese examined patent application publication No. SHO-43-443    Patent Reference 2: Japanese unexamined patent application publication No. HEI-5-114384    Patent Reference 3: Japanese unexamined patent application publication No. HEI-8-148114